1. The Field of the Invention
The present invention relates to the design and method of use of a bone plate and fastener implant and instrumentation system for stabilizing multiple bone segments. In one embodiment of this invention the system aligns and maintains adjacent human cervical vertebrae in a selected spatial relationship during spinal fusion of the cervical spine from the anterior aspect of the vertebrae.
2. Related Technology
The use of fixation plates and fastener systems for the treatment of spinal disorders for fusion of vertebrae has progressed considerably over the past twenty years. These systems usually include bone fasteners and plate systems that stabilize bone segments. The fasteners typically have a head, a shaft and threads that engage with the bone. The bone fasteners are placed by delivery mechanisms into corresponding openings in the plates and then into the bone itself. The fasteners are then firmly tightened to secure the plate to the bone.
A common problem associated with the use of such fixation plates is the tendency of the bone fasteners to back out of the plate under the dynamics of human movement. As a result of backout, bone fasteners may loosen and eventually disengage from the bone plate resulting in poor fixation. Potentially, this loosening of the bone fastener at the bone plate interface may cause the fastener to ultimately work itself out of both the plate and the bone from which it was implanted. This problem is particularly of concern in areas such as the spine where a loose fastener may impinge or interfere with sensitive tissues and bone structures.
Designers of such bone fixation systems have employed various techniques and developed different backout-preventing mechanism in an attempt to overcome the problem of fastener backout. These systems include secondary backout-preventing mechanisms and passive backout-preventing mechanisms. In secondary backout-preventing mechanisms, the bone fastener is first affixed into the bone through an opening in a bone plate. Once the fastener is in place, the secondary backout-preventing mechanism is then activated to secure the fastener to the plate. These secondary backout-preventing mechanisms comprise devices that are activated independently from the mechanism used to place the fastener. These mechanisms include secondary locking screws, locking collars, deformable tabs or other secondary locking devices that hold the bone fasteners in place after deployment within the plate and bone. The secondary backout-preventing mechanisms are typically independently activated in such ways that the mechanism limits the movement of the head of the bone fastener with the plate. This results in the fasteners being restrained by both the plate and the bone, thus lessening the likelihood of fastener backout.
For example, some designs found in the related art disclose an anterior cervical plating system incorporating an independent locking screw that engages the head of a bone fastener to secure the cervical plate to the vertebra. The locking screw, positioned above the bone fastener after the bone fastener is placed, provides a rigid fixation of the fasteners to the plate.
Other examples of designs found in the related art of secondary backout-preventing mechanisms include a threaded screw nut for use with a bone fixation system wherein the screw nut is partially insertable into an opening of the fixation plate, from the plate underside, and engages a portion of the bone fastener to thereby secure the bone fastener to the fixation plate after the fastener has been independently placed.
Further examples of designs for secondary backout-preventing mechanisms found in the related art disclose a bone fixation system wherein the head of the bone fastener is hollow and expandable. After the fixation plate is secured to the underlying bone by the hollow head bone fastener, a setscrew is then advanced into the hollow head of the fastener to radially expand the head and thereby secure the head to the fixation plate.
The successful use of such secondary backout-preventing mechanisms in the anterior cervical spine is particularly difficult because of the limited operating space available to the surgeon due to anatomic constraints. The above discussed secondary backout-preventing mechanisms require instrumentation to enter the surgical site and activate the backout-preventing mechanism. The instrumentation needed to activate these secondary backout-preventing mechanisms occupies space in the surgical site. In addition, the implementation of these mechanisms can be technically demanding and time consuming. To address the issues related to the limited space available for tools to activate secondary backout-preventing mechanisms and ease of use of the system, fastener and plate systems have been developed that incorporate passive backout-preventing mechanisms. These passive backout-preventing mechanisms are easier to activate since they typically deploy automatically while the surgeon drives the fastener into the opening in the plate and into the bone segment. Usually, no additional steps are required to fix the fastener to the plate. These systems include designs that lock the fastener to the plate by-either passively overcoming interference between the fastener and the plate or activating a passive spring like mechanism in the plate that locks the fastener to the plate.
For example, a bone fixation system wherein the head of the bone fastener is frustoconical in shape and has a directionally corrugated outer surface, is found in the related art. Wherein each opening in the fixation plate has a complementarily corrugated inner surface and is similarly frustoconical in shape. As the fastener is advanced through the corrugated openings and into the underlying bone, the direction of corrugation in the head and in the plate opening permits the head to be received within the corresponding opening, while inhibiting rotation of the fastener in an opposite direction.
Other passive mechanisms that are designed to prevent backout include a system in which a split ring is pre-mounted and attached to the plate. The split ring in the plate that retains the fastener to the plate by engaging the split ring with a groove in the fastener head, or the top of the fastener head. As the groove or the top of the fastener head aligns with the split ring, the split ring expands then snaps into the groove or over the top of the fastener, preventing the fastener from backing out.
Due to the potentially high loads between the plates and the fasteners, the backout-preventing mechanism retaining force need be maximized. While the above described passive backout-preventing mechanisms found in the related art can restrain the fastener to the plate and limit backout, the force required to overcome these mechanisms is typically small, within the magnitude similar to the force needed to drive the fastener into the backout-preventing mechanism. This is because the backout-preventing mechanisms are deformed by the fasteners as the fasteners are driven into the openings in the plate and deformed again in the reverse direction when the fasteners are removed from the openings in the plate. Thus, the force required to remove the fastener from the plate is similar to the force initially used to insert the fastener. Unfortunately, the backout force that the passive backout-preventing mechanisms are capable of restraining may be less than is clinically required for specific high load conditions.